1. Field of the Invention
The present invention relates to a device for moistening a running web of material by steam exiting from a housing having a treatment side that includes steam exit openings facing the web. The housing has a front side which faces the approaching running web. The invention further relates to a process for moistening a running web with the aid of at least one jet of steam being directed against the web of material.
2. Description of the Related Art
Devices and processes are used in the paper industry for moistening a paper web, prior to the web being guided through the nip of rollers of a roll apparatus. Due to the moistening, various parameters of the paper web, such as sheen and smoothness, can be influenced.
Such a device is shown in DE 43 01 023 A1 which, because of its wedge-like shape, can be placed relatively close to and in front of a nip of rollers of a roll apparatus. The treatment side of such a device does not necessarily have to be planar. In fact, depressions, into which the steam exit openings are placed, can be provided in the device. For reference purposes, the front side of this device faces the direction from which the material web is approaching. When the device is used in connection with a roll apparatus, the front side is the side facing away or remote from the roll apparatus.
The moistening of the web material with steam has proven to be an effective way of evenly moistening the web. However, there are still problems involved. For example, when the steam penetrates the web it must first pass through an air layer clinging to the web material. In most cases, it is important that the moistening is controlled such that the steam will condense after it has penetrated into the web of material and not before. If the steam were to condense before it penetrates into the web, the steam will collect on the web in the form of droplets. One way to achieve an effective steam penetration is to increase the speed or flow rate of the steam so that the energy of the steam is of such a magnitude as to enable the steam to penetrate through the clinging air layer. The higher the desired speed of the steam, the higher the steam pressure has to be, which results in a higher energy expenditure when operating the apparatus. A further disadvantage is created in that the steam is not fully absorbed by the web of material but is partially deflected therefrom and is diffused into the surrounding atmosphere.